People use the symbol to contribute to discussions, follow world events, and let the world know where they're tweeting from. According to Twitter, 125 million hashtags are tweeted every day on average.

how do you feel about using # (pound) for groups. As in #barcamp [msg]?

Hashtag use has grown considerably. In 2007, the most tweeted hashtag was used about 9,000 times. This year, the most popular hashtag has been used over 300 million times.

Some of the most popular hashtags have ushered in an entire lexicon of phrases. #ThrowbackThursday has been tweeted 120 million times, and people have showed off a #ootds, or outfit of the day, over 2 million times.

Hashtags get even more mileage during live events like big award shows, when people watch together and weigh in on the results on Twitter. Together, #MTVHottest, #MTVStars, #KCA, #iHeartAwards, and #BestFanArmy have been used over 3 billion times.

People also love to tweet about their favorite TV shows. Twitter says the most tweeted TV show of all time is "The Walking Dead" (#TheWalkingDead), though we like to think "Game of Thrones" (#GOT) will soon lap it.

In celebration of its birthday, Twitter created a 24-hour custom emoji. To check it out, just tweet #Hashtag10.